APRS is the Automatic Packet Reporting System. It’s a system for exchanging brief packets of information. It is most frequently used for mapping applications, but it really does a lot more than that. It has its biggest home in the amateur radio world, but isn’t limited to that, either.

The most common way to use APRS is to have some device hooked up to a GPS transmit packets with the GPS information in them. These packets can then be plotted on a map in real-time or with history. That in itself isn’t particularly newsworthy these days.

An interesting thing about APRS is that it’s not just positioning. Let’s say that there was a search-and-rescue operation. A person could draw a rectangle on the map indicating the search area, and within about 3 seconds everyone else’s map also shows that rectangle. People have even been known to play chess by sharing and moving objects on APRS!

The next piece that makes this interesting is that APRS is an ad-hoc mesh network. In its traditional implementation, VHF amateur radio, a radio emits a packet with a geolocation in it (a “beacon”) and any other radio within direct range of that can receive it. Radios can display basic information (such as distance to the other radio, heading, etc.) or hooked up to a laptop or mapping device for a better display. So if everyone is within a few miles, APRS works without any pre-existing infrastructure at all. This makes it wonderful for use in disaster areas, and was put to heavy use in Joplin after the tornado there.

But what about radios that are too far away? Any APRS station could also be a digipeater. When a packet is transmitted, it has a maximum hop count. A digipeater hears the packet, decrements the maximum hop count, and re-transmits it. With this mechanism, packets can travel hundreds of miles. It creates a highly resilient network, one that can route around trouble without even having to have an explicit backup route. I could bring in a digipeater in my car — it can be small enough to hold in my hand — and instantly improve APRS reception in an area.

One interesting aspect is that packets can be digipeated more than the maximum hop count. For instance, if a packet leaves my radio and is picked up by a digipeater to my west and one to my east, it can keep on traveling in both directions. This is part of what leads to resiliency.

APRS also functions over the Internet. There is a large network of interconnected Internet servers that exchange all global APRS traffic amongst themselves. Gateways between the radio (RF) and Internet (APRS-IS) services exist, and are called iGates. They are not generally required, but make useful websites like aprs.fi and email gateways possible. As long as an iGate is within a reasonable number of hops from you, you’re effectively linked. And again, if one iGate drops off, another iGate is probably monitoring your traffic too and you never notice. It’s an ad-hoc mesh network that is actually reliable – how about that?

On the PC side, there are many programs for using APRS. The most common one for Windows is called UI-View, but I don’t use Windows so I can’t comment. On Linux, there are programs (such as aprx) for running your digipeater, but the best-known program is Xastir. Xastir lets you download map files to your local disk, and can interface with the APRS-IS Internet service, radios, weather stations, or simply other arbitrary machines to exchange information. Xastir is a very nice program and is well worth the install, despite its somewhat dated-looking interface.

APRS clients, such as APRSDroid, exist for Android and iOS platforms as well.

So let’s say you’re doing something like helping handle food/water stations for a long bike ride. Even if you don’t have anybody with an amateur radio license, you can use APRS to great effect. At your headquarters, you can run Xastir and turn on its “server mode”. This puts everyone on a map. Then you can have everyone turn on APRS on their phone, and have it report to your custom server instead of APRS-IS. Now you have instant visibility into your entire team’s location and status. If you have transport people driving supplies between locations, that’s especially helpful.

In an amateur radio scenario, you would instead have people with radios at each location, and one laptop hooked up to a radio at HQ. This provides an added bonus of not relying on third-party infrastructure such as cellphone towers.

APRS also has a messaging system, similar in concept to text messaging. It works the same as other things. If I want to send a message to Jane, my radio simply emits a packet that lists the message and Jane as a recipient. It’s digipeated up to its maximum hop count. If Jane is within RF range of one of those digipeaters, she gets the message and her radio ACKs it. Otherwise, it’s delivered into the APRS-IS network — probably several times, which isn’t a problem — and the APRS-IS network delivers it to the iGate closest to her, and from there it gets digipeated the rest of the way to her.

Here’s an example of something created with APRS. While I was on a bus choir tour last weekend, I had a radio with me that was beaconing all the while. Now it was a small handheld radio inside a large metal bus, so it didn’t always have a digipeater in range. But still, you can go see a detailed map with the trail and even see exactly what path each packet took before it hit the Internet.

If you want to try out Xastir, please grab at least version 2.0 – the version in squeeze has some bugs.

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Nicely written John. Maybe I’m missing something in regards to APRSDroid. Have you been able to send or receive messages from a droid power device. I have only been able to send my “status” message with every beacon (ie monitoring 147.33+). Maybe I am expecting too much from my Droid 2.